Fuels represent a crucial energy supply and an important revenue source. Based on their provenience and quality (e.g., different grades or types of fuel), fuels can be differentially priced, such as taxed fuel and subsidized fuel or tax-free fuel; kerosene; diesel fuel; low-octane gasoline; high-octane gasoline; etc. Fuels can be differentially priced for a variety of reasons. In some countries, liquid fuel, such as diesel fuel, kerosene, and liquefied petroleum gas, is subsidized or sold below market rates to provide more widespread access to resources. Fuel can also be subsidized to protect certain industry sectors, such as public transportation.
Fuel adulteration is a clandestine and profit-oriented operation that is conducted for financial gain, which operation is detrimental to the rightful owner. Sometimes, fuels can be adulterated by mixing together fuels from different sources to obscure the origin of one or more of the fuels. Other times, adulterated fuels can be obtained by mixing higher priced fuel with lower priced fuel (e.g., lower grade fuel) or adulterants such as solvents. In some cases, subsidized fuel can be purchased and then re-sold, sometimes illegally, at a higher price. For example, subsidized fuel can be purchased and then mixed with other fuel to disguise the origin of the subsidized fuel.
Fuel markers can be added to fuels to establish ownership and/or origin of fuel. However, some markers placed in fuel for authentication can sometimes be at least partially removed to disguise the origin of the fuel. While some methods have employed the use of deuterated structures as fuel markers, such methods do not use of deuterated isotopologues to improve the accuracy of analysis.
Fuel adulteration can be assessed by determining the presence and concentration of fuel markers in a fuel sample via a variety of analytical techniques, such as gas chromatography (GC), mass spectrometry (MS), etc. Fuel markers can interact with their immediate environment (e.g., matrix), such as fuel, solvent, masking agents, etc., surrounding the marker, and the effect of the matrix can hinder the analysis of a fuel sample for determining whether a fuel is adulterated or not. While most prominently reported in trace level analysis of pesticide residues, matrix effects have been attributed to matrix components which cannot be efficiently separated from analytes of interest via a specified sample preparation methodology.
There have been a variety of approaches to mitigate matrix effects, such as for example pulsed inlet conditions, matrix matched standards, inclusion of analyte protectants in analytical samples, etc. While these approaches could improve detectability of target analytes (e.g., fuel markers) in some instances, their application to routine analyses proves rather complicated from a practical point of view. These approaches are usually neither generally applicable to a wide variety of chemical classes of fuel marker, nor desirable as they would add significant cost, time and complexity to the analysis.
Another approach to mitigate matrix effects can employ “matrix matched standards,” where standards can be prepared from the same matrix to be analyzed. While this approach can represent a way to reliably correct for matrix effects, a diverse matrix in combination with the lack of a priori knowledge of what problematic components are present in the matrix can prevent this approach from being an effective solution.
Yet another approach to mitigate matrix effects can employ analyte spiking (also known as “method of standard addition”), which entails adding known amounts of a standard to one or more aliquots of an unknown sample. This approach can generate a standard curve where the y-intercept of the linear regression fit of the collected data represents the endogenous concentration of the analyte (e.g., fuel marker) in the sample. While theoretically this approach could work, practically it entails too high of a cost in terms of time (requires 2-4 additional analyses per unknown sample) and the analysis of the data can be considered too complex for the practical purpose of fuel authentication.
Existing analytical approaches to determine fuel adulteration and mitigate matrix effects all have significant limitations that preclude their utility in fuel authentication. Thus, there is an ongoing need to develop and/or improve fuel markers and methods for detecting these markers.